You are bitten by autovivification:

while ( $eventscores->[$i]->[0] == $rider_id ){
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If $i at this time points beyond the end of the array, an arrayref will spring into existence, as you dereference the hitherto unexisting element.

Solution: Don't do that. A common strategy is to protect the dereferencing:

while ( $eventscores->[$i] and $eventscores->[$i]->[0] == $rider_id ){
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Depending on the rest of your outer loop, other solutions may be cleaner though.